fix(briefing): use briefing context for follow-ups; add slot separator

- build_context: when conversation_type is 'briefing', inject a system
  prompt instruction telling the model to answer from conversation history
  and article context instead of searching the web
- Consolidate briefing conversation type detection to one DB query (was
  being checked twice — once for the system prompt addition, once for
  article context injection)
- ChatPanel: render a visual 'New Briefing Update' separator line before
  2nd+ briefing slot messages (identified by metadata.rss_item_ids)
- types/chat.ts: add metadata field to Message interface

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/types/chat.ts

@@ -28,6 +28,7 @@ export interface Message {
   context_note_id: number | null;
   context_note_title?: string | null;
   tool_calls?: ToolCallRecord[] | null;
+  metadata?: Record<string, unknown> | null;
   created_at: string;
   timing?: GenerationTiming;
   thinking?: string;